Understanding Linux Network Internals

Tuning via /proc Filesystem

The IPv4 routing subsystem uses the /proc filesystem to export some internal data structures in read-only mode (e.g., the cache), and other structures in read-write mode so that they can be used for tuning.

Figure 36-4 shows where these files are located and the routines that register them. The files shown without a reference to a creating routine are statically defined by sysctl_init at boot time.

/proc/sys/net/ipv4/

/proc/sys/net/ipv4/conf

/proc/sys/net/ipv4/route

These directories are used to export internal data structures used for tuning. The files in these directories are therefore writable. Later sections list their files, the associated kernel variables, and the variables' default values when applicable.^[*]

/proc/net/

/proc/net/stat

Files in these directories are not used for tuning, but rather, to execute kernel routines to get some kind of information. See the section "The /proc/net and /proc/net/stat Directories."

Figure 36-4. /proc files used by the IPv4 routing subsystem

The /proc/sys/net/ipv4 Directory

This directory contains a lot of files, but the only ones used by routing subsystems are:

ip_forward

Contains a Boolean flag that can be used to globally enable and disable IP forwarding. Its value can be overwritten on a per-device basis (see the section "Enabling and Disabling Forwarding").

icmp_echo_ignore_broadcasts

An ICMP tuning parameter. ...